Field of the Invention
The present invention relates to control of a robot arm including an encoder.
Description of the Related Art
Robots having an articulated robot arm and an end effector provided at an end of the robot arm are used in production lines for manufacturing products (parts). A joint mechanism of a robot arm includes a servo motor, such as an alternating-current (AC) servo motor and a direct-current (DC) brushless servo motor, and a reduction gear which is arranged on an output side of the servo motor to obtain high output torque. The servo motor and the reduction gear are connected to structural members, such as a link. An encoder (hereinafter, referred to as an “input encoder”) is directly connected to an input side of the reduction gear, that is, a rotating shaft of the motor, to detect an angle. Position control on the end of the robot arm (the hand end of the robot) is performed based on the result of detection.
In the control using the input encoder, torsion or backlash of the reduction gear is not detected. A position error at the end of the robot arm is therefore occurs. Some driving systems use a timing belt between the motor and the reduction gear. Torsion and backlash ascribable to the timing belt can also cause a position error at the end of the robot arm. A change in the orientation of the robot arm and the weight of a workpiece can also cause a position error at the end of the robot arm.
Japanese Patent Application Laid-Open 2011-123716 discusses a configuration in which an encoder (hereinafter, referred to as an “output encoder”) is arranged on an output side of the reduction gear, and a configuration in which encoders are arranged on both the input and output sides of the reduction gear. The use of the output encoder for position control on the end of the robot arm can reduce the position errors at the end of the robot arm.
There are two types of encoders: absolute encoders and incremental encoders. Both the encoders include a patterned scale and a detection head. The detection head moves relatively with respect to the scale and reads a position from the pattern of the scale.
If a defect, such as a scratch, is formed on the scale of an encoder or dust adheres to the scale, the detection head can be unable to detect an accurate detection position. In other words, a read error can occur.
Robots are mostly installed on a production line of a factory. If a robot is stopped because of a read error, the production of parts stops for the same period. To resume the production of parts after a stop of the robot, recovery operations, such as disassembling the robot arm and removing dust from the encoder or replacing the encoder, or replacing the robot arm, are needed.
If the robot arm is disassembled, new dust may enter the encoder depending on the cleanliness inside the factory. If the robot arm is replaced, re-teaching (setting of operation positions of the robot) is needed because of the presence of mounting errors and manufacturing variations of the robot arm. This means that a long time is required before recovery. In any case, when a read error occurs, robots are stopped. Since the production line needs to be frequently stopped for recovery operations, the production efficiency has been low.